My friends and I are getting the very same issue. I am playing PC version, and we're all playing the Definitive Edition.
I play with two friends -- but in two separate playthroughs, each with a friend.
So that means that in either playthrough, there are only two human players (me and a friend) and the other two party members are AI companions we met and invited to come with us. My friends and I are using custom characters.
With friend A (who is on a PC), he is the one hosting the game. During party conversations, it seems his character always says his line first (and successfully voices it). But when I pick a dialogue choice for my character, it immediately skips right to an AI companion's choice (which gets voiced).
With friend B (who is on a Mac), I am hosting the game. Same issue, except that it seems my character always gets to say his line first (successfully), while my friend's choice doesn't get voiced (again skips to AI companion's choice which gets voiced).
So far, it seems that it has to do with who is hosting the game (host seems to always get their line in first), but more importantly the AI behaviour because unlike players they don't wait to listen to any audio, and require virtually no time to "make a dialogue choice" while players would.
I imagine it could be easily fixed if, during the time the character is meant to be voicing their line, there is a "continue" button that player needs to click on before it allows the AI companion their turn.
